Summary

Pesticides are among the most investigated priority pollutants in agricultural products due to their widespread use and toxicity. The olive fruit fly (Bactrocera oleae) is one of the main pests of the olive tree in Portugal, and for an effective protection against this plague, farmers rely mostly on the use of insecticides. Six olive samples were collected in four olive groves to evaluate the presence of residues of dimethoate used for the combat of olive fruit fly, in table olives at the time of harvest and after brine. QuEChERS (Quick, Easy, Cheap, Effective, Rugged and Safe) extraction was optimized, one factor at a time, applied to the olive samples, and the extracts were analysed using liquid chromatography–diode array detection. Positive results were obtained in two samples at the time of harvest (3.58 and 4.34 mg/kg fw). One olive grove was selected to test two concentrations of salt and assess their influence on the quantity of residues. After brine, dimethoate residues were not detected.